The present invention relates to a vibration drive for a machine for producing shaped concrete bodies from a mold which is placed on a vibrating table and filled with flowable concrete, more particularly, to such a vibration drive utilizing one or more piezoelectric vibration exciters connecting a vibrating table to the frame of the machine.
In such vibration drives, cam and eccentric motors have generally been used in order to cause the vibrating table of the molding machine to vibrate. As a result, the mold which is open at the top and bottom and which is positioned on the vibrating table is similarly caused to vibrate and shaken in order to compact the concrete mass which has been placed in the mold cavities as uniformly as possible. During the shaking process, the opened top of the mold is generally closed using vertically movable pressure plates which move downwardly from overhead into the mold cavities and press on the concrete mass.
The disadvantage in such known structures is that the mechanical eccentric motors provide largely uncontrolled shakey movements which can lead to damage and premature wear phenomena of the mold. Therefore, the mold and the vibrating table must be built to be very strong and is thus more complex. In addition, the mold machine and the mold are often not optimally matched to one another with respect to vibration engineering. The same also applies to the concrete mass which has been placed in the mold and which, depending on the type, volume, grain size, moisture content, specific weight and other properties requires different vibration parameters such as vibration frequency, vibration duration, vibration path, vibration direction and others. These discrepancies of matching leads to nonuniform filling of the mold cavities and to nonuniform compaction of the concrete mass within the mold. As a consequence thereof, the finished moldings are of relatively poor quality. Also, the molds which must be made thicker and the relatively heavy vibrating table also require much higher vibration energy.
The German published patent application DE OS 38 37 686 discloses a three-dimensional vibration system in which a mold which is filled with a concrete mass is kept in resonant vibration to produce the concrete mold bodies. The mold is supported by bearing springs on the machine frame and is caused to vibrate by means of vibration exciters in the form of eccentric motors. Sensors indicate various parameters such as the stiffness and damping of the bearing springs and the resonant frequency of the vibration system is measured and monitored in a microprocessor. When the resonant frequency is exceeded or not attained, a corresponding correction occurs by changing the bearing spring parameters in order to keep the vibration system at the desired resonant frequency. In this manner, optimum vibration conditions will be created with low input power.
This known prior art structure which has not yet been put into practice has the disadvantage that conventional mechanical eccentric motors are always used as the vibration exciters, but they are not especially well-suited to the control of the exciter frequency. The additional construction which is required to control the exciter frequency by changing the bearing spring parameters is very considerable. At the beginning and end of the shaking process the mechanical eccentric motor traverses a rpm. range from zero to maximum and back again. In so doing, the individual components or groups of components are briefly excited to the natural frequency. This results in damage and additional noise. Further, the cycle time of the molding machine is considerably lengthened.
It is the principal object of the present invention to provide a novel and improved vibration drive for a machine for producing shaped concrete bodies from a mold.
It is another object of the present invention to provide such a vibration drive which can be readily adjusted according to the various requirements of practice in order to ensure optimum vibration behavior of the mold and thus high quality of the final product.
The objects of the present invention are achieved and the disadvantages of the prior art as described above are overcome by the vibration drive of the present invention which has at least one piezoelectric vibration exciter with a stationary portion which is connected to the machine frame and a vibrating member which is connected to the vibrating table.
In order to directly transmit vibrations, a vibrating member of the vibration exciter is connected to a piezoelement through a transducer which may comprise a hydraulic fluid. The piezoelement is clamped in the stationary portion of the vibration exciter so as to be able to vibrate freely. A first or larger piston is connected to the piezoelement and vibrations are transmitted through a hydraulic fluid to a second or smaller piston which is attached to the vibrating member of the vibration exciter. Downward or return movement of the second piston can be enhanced by a return spring. The piezoelement may be a ceramic.
In order to achieve a simple, vertical shaking motion of the mold in one embodiment there is provided one vibration exciter in each of the four corner areas of the rectangular vibrating table and each exciter has a vertical vibration direction.
Another modification of the present invention provides for three-dimensional vibration of the mold and different directions of vibration. This is achieved by mounting a vibration exciter in each of the four corner areas of the vibrating table but with the longitudinal or vibration axis of each exciter being at an angle with the vertical which angle is preferably 45 degrees. It is preferable that the vibration exciters are connected by spherical bearings to the vibrating table and/or the machine frame. These spherical bearings preferably consist of ball- and socket joints.
A further modification of the present vibration drive can be applied to a molding machine having pressure plates which correspond to the mold cavities and which can be moved vertically by lifting elements from overhead to press on the concrete mane in the mold cavities. To reinforce the vibratory motion one or more vibration exciters may be interconnected between the pressure plate lifting elements and the pressure plates. As a result, an additional vibratory or shaking motion is applied through the pressure plates to the concrete mass and the distribution and compaction of the concrete mass and the mold cavities are significantly improved.
In order to obtain optimum vibratory action under different operating conditions, parameters of the vibration drive such as the vibration frequency, vibration duration, vibration path, vibration direction and the number of activated vibration exciters can be varied. This can be accomplished by connecting the vibration exciters to a microprocessor which contains one or more preselectable programs for adjusting the required parameter quantities of the vibration drive. The vibration exciters can be activated or controlled individually or in predetermined groups.
The advantages and results achieved with the present vibration drive is that the utilization of the piezoelectric vibration exciters enables one to vary the vibration drive with different operating conditions, depending on the product, and characteristics of the concrete mass to result in a high quality bolded concrete body. The use of the piezoelectric vibration drive as disclosed herein instead of the conventional eccentric motors makes it possible to generate the desired exciter frequency immediately and relatively easily. The energy required to produce vibrations is significantly reduced and the noise of the process is greatly decreased. Further, the vibration metal bearings which are located between the vibrating table and the vibrating frame in a conventional vibratory machine and which absorb a large part of the exciter frequency are eliminated in the present invention. Further, since the vibration exciters of the present invention also support the vibrating table, an empty space is formed underneath the mold which can be used for inserting mold cores or recess bodies into the mold.